1. Field of the Invention
The invention relates to underground storage tanks generally and more particularly to a double walled underground storage tank in which the two walls are monolithically tied together to act as a composite structure.
2. Discussion of the Background
Underground storage tanks are used in a wide variety of locations to store materials underground. The materials are often harmful to the environment. Examples of such materials include gasoline and other petroleum products as well as toxic raw materials and waste from manufacturing processes. Because of the harmful nature of these materials, it is especially important to ensure that underground storage tanks containing such materials do not leak or release these materials into the environment.
Concern over this possibility has lead many governmental authorities to require secondary containment for tanks that store such materials. As used herein, secondary containment means that there are at least two barriers that prevent materials in the underground storage tank from being released into the surrounding soil. There are several ways in which secondary containment can be provided. For example, one way to provide secondary containment is to locate underground storage tanks in a vault. In this manner, should the tank itself break, material leaking from the tank will be contained by the vault. One of the most the most common methods for providing secondary containment is through the use of double walled underground storage tanks.
The first double walled underground storage tank prepared from corrosion resistant materials such as fiber reinforced plastic (FRP) was introduced by the Assignee of the present application in 1984. A diagram of this tank, which was referred to as the xe2x80x9cDouble Wall Ixe2x80x9d or xe2x80x9cDWT Ixe2x80x9d tank, is shown in FIG. 6. The tank 6000 included an inner wall 6100 and an outer wall 6104 separated by a plurality of ribs 6102. An annular space 6101 was created between the inner wall 6100 and the outer wall 6104. This annular space 6101 was quite large, which increased the cost of the tank, especially when a monitoring fluid was used in the annular space 6101 to detects leaks. The volume of the annulus also imposed the requirement that it be filled at the installation site, rather than being filled at the factory. Additionally, the inner wall 6100 and the outer wall 6104 were not tied together in any fashion. This impacted the cost of the DWT I tank because each of the walls 6100, 6104 were required to be stronger that in the situation where the two walls could share strength locally as well as globally.
In order to provide the strength and safety of the Double Wall I tank at a reduced cost, Xerxes Corporation commercially introduced a new double wall tank referred to as the DWT II tank. This tank modified the original DWT I design by applying a faced plastic netting or spacer to the interior surface of the outer wall which is first formed wall in a female molded tank. This netting was xe2x80x9clocked into placexe2x80x9d with additional fiberglass material. Thereafter, an interior wall was formed, and bonded to the outer wall. The resulting tank was strong, durable, and environmentally sound but required considerable expertise to manufacture. Although the size of the annulus in the DWT II tank was greatly reduced as compared to the DWT I tank, it was still thicker than necessary for monitoring purposes. Ideally, only the minimum space necessary to provide for flow of liquid between the two walls should be provided.
An alternative approach to the DWT II tank is illustrated in U.S. Pat. No. 5,220,823. In this approach, a liquid pervious mat or felt is placed between the two walls. This material transmits load between the two walls. Because load is transmitted between walls locally as well as globally, each wall may be of reduced thickness. While this approach has independent advantages as set forth in that commonly assigned patent, it again presents an annulus of significant thickness and, particularly in the manufacture of a female molded tank, requires application of material to the interior of the tank and adherence of the material to the interior of the external wall, followed by application of the interior wall in a way that it adheres to the interior surface of the material. This generally requires use of additional adhesives, which complicates a process which already employs sprayed resin and catalyst.
A significant improvement to double walled underground storage tanks is described in U.S. Pat. No. 5,720,404 (xe2x80x9cthe ""404 patentxe2x80x9d), the contents of which are incorporated by reference herein. The ""404 patent is also owned by Xerxes Corporation, the assignee of the present application. In the tank described in the ""404 patent, the size of the annulus between the inner wall and outer wall is greatly reduced through the use of a thin plastic film, such as Mylar(copyright). In addition to greatly reducing the size of the annulus through the use of Mylar(copyright), the tank described in the ""404 patent includes the additional advantage of having the inner wall and the outer wall tied directly together, rather than through an annular material such as was done in the tank described in the ""823 patent. However, the tank described in the ""404 patent required gutters to be installed in order to ensure communication between all areas of the annulus. The provision of gutters requires additional manufacturing steps, which leads to increased cost.
What is needed is a storage tank that can realize the benefits of the tank discussed in the ""404 patent without requiring the provision of gutters.
The aforementioned need is met to a great extent by the present invention which provides an underground storage tank including an outer wall having integrally formed ribs, the ribs being closed off by a layer of material applied directly over the space defined created by the ribs and attached to the outer wall, a layer of plastic film or other annular material which facilitates liquid flow placed over portions of the outer wall between the ribs and extending past the shoulders of the ribs (as used herein, xe2x80x9cshoulderxe2x80x9d refers to the location where the ribs are joined to or integrally molded with the outer wall or where the outer wall curves to form the ribs) but not extending completely over the material covering the opening of the rib, and an inner wall formed over the annular material and attached to the portion of the material covering the opening of the rib that is not covered by the annular material. In this way, the inner wall is bonded to the outer wall through the material covering the opening of the rib. In preferred embodiments, the material covering the opening of the rib is applied before the outer wall has fully cured, and the inner wall is applied before the material covering the opening of the rib has fully cured, so as to form primary bonds and function as monolithically formed. At one or more locations along the circumference of the tank, a strip of annular material is placed over the material covering the rib, and a hole is formed in the material covering the opening of the rib such that communication between all annular areas of the tank (the annular areas between ribs formed by the annular material as well as the annular spaces defined by the ribs) is established. Functionally, this replaces the aforementioned gutters of the ""404 patent tank and is much easier to perform.
The aforementioned advantages and features of the present invention will be more readily understood with reference to the following detailed description and the accompanying drawings in which:
FIG. 1 is a side view of a portion of an underground storage tank according to the present invention.
FIG. 2 is an end view of the storage tank of FIG. 1.
FIG. 3 is a cross-sectional view of a section of the tank of FIG. 1 taken along the line Axe2x80x94A of FIG. 1.
FIG. 4 is a cross-sectional view of a second section of the tank of FIG. 1 taken along the line Bxe2x80x94B of FIG. 1.
FIG. 5 is a cross-sectional view of a portion of the tank of FIG. 1, taken along the line Cxe2x80x94C of FIG. 2.
FIG. 6 is a cross-sectional view of a double wall underground storage tank according to one form of the prior art.